Garrosh HM don't drop a wolf, we don't know what he drops. It's the garrosh normal 5.4 achiev who give a wolf. And IMO this mount can be a son of gallion or onyx celestial cloud serpent like mount, droping on the 4 celestials.

Looks like an Anzu/Druid Flight Form hybrid, maybe a Druid only quest similar to the Warlock quest for green fire?
It makes zero sense for this mount to drop from Garrosh unless there's some crazy ass lore pulled out to explain why Garrosh has a Storm Crow hanging around.